The present invention relates to phase shifting circuits in which the phase of a signal may be changed in response to a control input and, more specifically, to circuits for use in high frequency applications for switching the phase of a signal by 180.degree..
Many types of electronic equipment require the phase of an input signal to be shifted or switched by 180.degree.. In particular, for certain high frequency applications such as adaptive antennas and bi-phase modulators, phase switching circuits are required which are of small size, low component count and low cost. Furthermore, these circuits must feature low noise figures, constant input impedance, and consistent high gain both between phase positions and over extended frequency ranges.
Referring now to FIG. 1A, a typical prior art split-load phase invertor circuit 10 of the type commonly used in high frequency applications is shown. An input signal RF.sub.IN is coupled over the line 11 to the gate of the field effect transistor or FET 12 while the drain of the FET 12 is connected through a resistor 13 to a source voltage V.sub.DS. The drain of the FET 12 is also coupled over the line 14 to provide a signal S2 and the source of the FET 12 is coupled over the line 15 to provide a output signal S1 and connected through resistor 16 to ground. In operation, the FET 12 conducts current in response to the voltage on applied to its gate causing voltage drops to occur across the resistors 13 and 16 and generating the signals S1 and S2 which are 180.degree. out of phase with respect to each other. The switch 17 selects between the signals S1 and S2 in furnishing the output RF.sub.OUT of the circuit 12. However, the split-load phase invertor circuit 10 does not provide low-noise or constant gain and requires 2 to 6 transistors to be implemented in an MMIC.
Referring now to FIG. 1B, a passive phase-shift circuit 20 also typical of the prior art is shown which includes phase-lag and phase-lead networks 21 and 22 comprising alternate signal paths between the input and output terminals of the circuit. A pair of switches 23 and 24 are used to direct an input signal RF.sub.IN through one or the other of the filter networks 21 and 22 and furnish an output signal RF.sub.OUT which may be switched 180.degree. in phase depending on the network 21 or 22 through which the input signal is directed. Alternatively, one of the networks 21 or 22 may simply comprise a 180.degree. long transmission line while the other network provides 0.degree. phase delay. In any event, the circuit 20 requires a great deal of space to implement the necessary switches, filters or delay lines and does not provide any gain. Further, the circuit 20 only performs the phase shift with a constant output signal over a narrow range of frequencies.
It is therefore an object of the present invention to provide a phase switching circuit for use in high frequency applications which is of small size and has a low component count so as to provide a simple interface for switching the phase of a signal by 180.degree..
It is another object of the present invention to provide a phase switching circuit featuring high gain characteristics and constant input impedance which are consistent regardless of phase switching.
It is a further object of the present invention to provide a phase switching circuit which provides consistent gain and phase characteristics over a range of frequencies and which can be produced at low cost.
It is yet another object of the invention to provide a phase switching circuit for use in conjunction with the adaptive antennas employed in GPS (Global Positioning System) communications at the L.sub.1 and L.sub.2 GPS frequencies.